Nitrogen oxides (hereinafter may be referred to as NOx) contained in exhaust gas from, for example, waste incinerators, engines, and electrical generators are atmosphere pollutants, so that their emission is strictly regulated. Therefore, the reduction of the concentration of nitrogen oxides contained in exhaust gas by providing a denitration means in an incineration facility or an internal combustion engine is becoming an essential technique.
In order to treat exhaust gas containing nitrogen oxides, a selective catalyst reduction method (referred to as the SCR method) using ammonia and a catalyst is commercially used. The SCR method is a technique of blowing ammonia into the exhaust gas at a temperature of about 400° C., and reducing nitrogen oxides into nitrogen and water on a catalyst. The denitration rate by the method is as high as 80% or more, and in Japan the method is mainly used in large burning facilities such as thermal power generation. In addition, the SCR method is also currently used in middle- and small-scale combustion furnaces where the concentration of nitrogen oxides is strictly regulated. However, the ordinary SCR method uses a costly catalyst, so that the whole facility is very costly. Furthermore, because of the life and abrasion of the catalyst itself, replenishment and replacement during use are necessary. Major shortcomings of the SCR method are the high costs of the initial investment and operation management for the catalyst. In a burning facility where the cost for environmental measures cannot be sufficiently secured, commercialization of a non-catalytic denitration method (referred to as the SNCR method) using no catalyst is strongly desired.
Patent Literature 1 discloses a basic structure of a non-catalytic denitration method. The non-catalytic denitration method disclosed in Patent Literature 1 is a method including injecting ammonia or ammonia generated from a substance generating ammonia (for example, urea) into a combustion furnace at a high temperature (800° C. to 1100° C.), wherein nitric monoxide (NO) is reduced to water and nitrogen according to the reaction formula:4NO+4NH3+O2→4N2+6H2O  (Formula 1)
FIG. 11 shows the result of validation of denitration characteristics of a prior art non-catalytic denitration method. The denitration reaction occurs in the temperature range from 800° C. to 1100° C. (hereinafter may be referred to as TW), and the temperature condition is particularly preferably about 900° C. In FIG. 11, the denitration rate used as the index of denitration properties is the rate of NO reduction, and defined by the following formula:(NO0−NO1)/NO0×100[%]  (Formula 2)wherein NO0 is the concentration (ppm, dry) of nitric monoxide contained in the burning exhaust gas, and NO1 is the concentration (ppm, dry) of nitric monoxide after denitration.
When the non-catalytic denitration method is used in an actual combustion furnace, on the basis of the above-described disclosure, ammonia is blown into the region where the exhaust gas temperature becomes 900° C. However, there are problems that the actual denitration rate by the non-catalytic denitration method is as low as about 30%, and the efficiency of denitration is lower than that by the SCR method, because the exhaust gas temperature in the combustion furnace is unstable and difficult to be kept at 900° C., and the retention time of exhaust gas at 900° C. is often shorter than 0.3 seconds in the combustion furnace and exhaust channel communicated therewith. Therefore, many researches are being conducted for carrying out denitration at a lower temperature with a higher efficiency.
Patent Literature 2 discloses a technique for expanding TW to the low temperature side by a method blowing hydrogen peroxide. Patent Literature 3 discloses a technique for expanding TW to the low temperature side by a method using an electron beam. Patent Literature 4 discloses a technique for expanding TW to the low temperature side by a method using corona discharge. Patent Literature 5 discloses a technique for expanding TW to the low temperature side by a method using pulse discharge. As a method having advantages over prior art in terms of the cost and performance, the inventors developed a device and a method for blowing ammonia modified by plasma, and disclosed the techniques in Patent Literatures 6 and 7.
However, it is pointed out that the denitration rate by the prior art SNCR method is still as low as about 60%. In addition, it has been pointed out that the increase of ammonia to be injected for the purpose of improving the denitration rate can cause the discharge of unreacted ammonia into the air. Therefore, demanded is a technique for stably carrying out denitration in a lower reaction temperature range with a higher efficiency.